1. Field of the Invention
The present invention relates to an improved memory system mounted directly on a motherboard and an associated method.
2. Description of the Related Art
Most computer systems allow for memory expansion using memory modules. Memory modules—for example, single inline memory modules (SIMMs) and/or dual inline memory modules (DIMMs)—are small, compact circuit boards that are designed to connect easily into an expansion socket mounted on a main circuit or motherboard.
FIGS. 1 and 2 are diagrams of computer system 100's memory architecture. Referring to FIGS. 1 and 2, a system 100 includes a plurality of memory modules 106 controlled by a memory controller 104. The memory controller 104 controls read and write operations relating to the memory modules 106. The memory controller 104 is mounted on the main or motherboard 102. The operation of the memory controller 104 is well known to a person of reasonable skill in the art and will not be discussed any further.
The memory modules 106 oftentimes include edge connectors 114 comprising a plurality of contact pads 116. The contact pads 116 are typically present at both sides of the modules 106. A plurality of receptacles, e.g., sockets 112, is mounted on the motherboard 102. The sockets 112 receive the edge connectors 114 to thereby electrically couple the motherboard 102 to the memory modules 106. More particularly, the sockets 112 electrically couple traces routed on the motherboard 102 to traces routed on the memory modules 106.
The memory modules 106 include a plurality of memory devices 108. These memory devices 108 are, for example, dynamic random access memory (DRAM) or synchronous dynamic random access memory (SDRAM). A buffer 110 controls and buffers commands and addresses (C/A) it receives from the memory controller 104. The plurality of memory devices 108 and the C/A buffer 110 are mounted on the memory module 106.
Signal traces are routed on both the motherboard 102 and the modules 106. These signal traces might include a data bus DQ, system clock signal CLK, and C/A bus. The memory devices 108 and the buffer 110 receive signals from a controller 104 through the corresponding socket 112 mounted on the motherboard 102.
In any memory architecture, it is important to maintain the signal integrity of the address, control and clock signals. Maintaining signal integrity becomes more difficult as the operating frequency increases because of transmission line effects, including signal reflection.
In transmission line theory, the connection between the motherboard 102 and the module 106 through the socket 112 is termed a stub load. Stub loads present a transmission discontinuity that results in signal reflection and ultimately, adversely affects signal integrity. Referring to FIGS. 1-3, a signal trace 302, e.g., the data bus DQ, is routed on the motherboard 102. The signal trace 302 is electrically coupled to a signal trace 304 routed on the module 106 through the socket 312. But the socket 312 presents a discontinuity 306 between the signal trace 302 and the signal trace 304. The discontinuity 306 causes a portion of the signal to reflect back injecting noise, as well as, decreasing timing margins and voltage windows.
Referring to FIG. 4, stub resistors 416 in the memory modules 406 reduces signal reflection. But these stub resistors 416 improve memory read and write operations. As the value of the stub resistors 416 increases to decrease reflection, the voltage drop across it increases attenuating the signal voltage. Attenuating signal voltages decrease the voltage window. And the stub resistors 416 might cause RC parasitic loads that delay the signal.
Accordingly, a need remains for a memory system capable of addressing disadvantages associated with existing memory systems.